Conventionally, a so-called laminated evaporator has been widely used as an evaporator for use in a car air conditioner. In the laminated evaporator, a plurality of flat, hollow members, each of which includes a pair of depressed plates facing each other and brazed to each other at their peripheral edge portions, are arranged in parallel, and louvered corrugate fins are each disposed between and brazed to the adjacent flat, hollow members. In recent years, evaporators have been required to be reduced further in size and weight and to exhibit higher performance.
The inventors of the present invention have proposed an evaporator which fulfills the above requirements (refer to Patent Document 1). The evaporator includes a heat exchange core section configured such that heat exchange tube groups are arranged in two rows in a front-rear direction, each heat exchange tube group consisting of a plurality of heat exchange tubes arranged at predetermined intervals; a refrigerant inlet/outlet header tank disposed on an upper-end side of the heat exchange core section; and a refrigerant turn header tank disposed on a lower-end side of the heat exchange core section. A partition wall divides the interior of the refrigerant inlet/outlet header tank into a refrigerant inlet header section located frontward of the partition wall, and a refrigerant outlet header section located rearward of the partition wall. A refrigerant inlet is formed at a first end portion of the refrigerant header section, and a refrigerant outlet is formed at an end portion, corresponding to the first end portion of the refrigerant header section, of the refrigerant outlet header section. A partition wall divides the interior of the refrigerant turn header tank into a refrigerant inflow header section located frontward of the partition wall, and a refrigerant outflow header section located rearward of the partition wall. A plurality of refrigerant passage holes are formed in the partition wall of the refrigerant turn header tank at predetermined intervals in a longitudinal direction. Upper end portions of the heat exchange tubes of the front heat exchange tube group are connected to the refrigerant inlet header section, whereas upper end portions of the heat exchange tubes of the rear heat exchange tube group are connected to the refrigerant outlet header section. Lower end portions of the heat exchange tubes of the front heat exchange tube group are connected to the refrigerant inflow header section, whereas lower end portions of the heat exchange tubes of the rear heat exchange tube group are connected to the refrigerant outflow header section. A refrigerant which flows into the refrigerant inlet header section of the refrigerant inlet/outlet header tank passes through the heat exchange tubes of the front heat exchange tube group to thereby flow into the refrigerant inflow header section of the refrigerant turn header tank; passes through the refrigerant passage holes of the partition wall to thereby flow into the refrigerant outflow header section; and passes through the heat exchange tubes of the rear heat exchange tube group to thereby flow into the refrigerant outlet header section of the refrigerant inlet/outlet header tank (see Japanese Patent Application Laid-Open (kokai) No. 2003-75024).
However, various studies conducted by the present inventors have revealed that enhancing heat exchange performance to a higher level is difficult for the evaporator described in the above-mentioned publication, for the reason described below.
As compared with the laminated evaporator, the evaporator described in the above-mentioned publication is likely to be greater in cross-sectional area of a channel within the refrigerant inlet header, so that channel resistance is likely to become lower. However, since the overall internal volume of the refrigerant inlet header section with which the heat exchange tubes communicate becomes large, response tends to become slow, particularly, at the time of on-off control of a compressor. Specifically, even when the compressor is turned on, much time may be consumed before the evaporator begins to be cooled, for the following reasons: since the overall internal volume of the refrigerant inlet header section is large, the flow velocity of refrigerant becomes low; and since the overall internal volume of the refrigerant inlet header section with which the heat exchange tubes communicate is large, the refrigerant does not begin to flow into the heat exchange tubes until the quantity of refrigerant within the refrigerant inlet header section builds up to a certain level. On the contrary, even when the compressor is turned off, distribution of temperature rises in the evaporator may become nonuniform with a resultant nonuniform temperature distribution of discharged air; i.e., a nonuniform temperature distribution of air that has passed through the heat exchange core section, for the following reason: since the overall internal volume of the refrigerant inlet header section is large, distribution of the quantity of refrigerant remaining within the refrigerant inlet header becomes nonuniform with respect to the direction in which the heat exchange tubes are arranged. Further, since the internal volume of the refrigerant inlet header section becomes large, at low refrigerant flow rate, the refrigerant which has flowed into the refrigerant inlet header section becomes unlikely to flow up to a distant location from the refrigerant inlet. In the front heat exchange tube group, a large amount of refrigerant flows into the heat exchange tubes located near the refrigerant inlet, so that the refrigerant flow rate in the heat exchange tubes becomes high; and a small amount of refrigerant flows into the heat exchange tubes located away from the refrigerant inlet, so that the refrigerant flow rate in the heat exchange tubes becomes low. Also, in the rear heat exchange tube group, the refrigerant flow rate in the heat exchange tubes located near the refrigerant inlet becomes high, and the refrigerant flow rate in the heat exchange tubes located away from the refrigerant inlet becomes low. As a result, distribution of the quantity of refrigerant contributing to heat exchange becomes nonuniform in the heat exchange core section with respect to the longitudinal direction of the refrigerant inlet/outlet tank, and the temperature distribution of discharged air becomes locally nonuniform, potentially resulting in a failure to sufficiently yield the effect of enhancing the heat exchange performance of the evaporator.
An object of the present invention is to solve the above problem and to provide a heat exchanger which exhibits excellent heat exchange performance, particularly when used as an evaporator.